The term “patterning device” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT patent application publication WO 98/40791, which are incorporated herein by reference.
In a lithographic apparatus it is essential that the substrate is held securely on the substrate table. The position of the substrate is required to be known accurately, even when the table undergoes high accelerations. In a conventional lithographic apparatus it is known to provide this clamping by means of a vacuum. The substrate table is provided with protrusions or pins and an outer wall. The substrate sits on top of these pins and the space behind it is evacuated. The action of air pressure on the surface on the substrate then serves to hold it securely in place. Further details of such a substrate holder can be found, for example, in European patent application EP-A-0 947 884, incorporated herein in its entirety by reference.
While such a clamp is effective in many applications, recent developments in lithography require the use of EUV (extreme ultraviolet) radiation, x-rays, electrons or ions. Unlike conventional lithography in which UV radiation is employed, EUV techniques require that the beam path, or at least a substantial part of the beam path, is evacuated. The vacuum clamping method is then less effective.
It has been proposed to provide a system of clamping for use in a lithographic apparatus which uses electrostatic forces to clamp the substrate to the substrate table. Such a system will work regardless of the presence of a vacuum on both sides of the substrate. In such a system, the substrate is either conductive, or provided with a conductive electrode on the surface which is to be attached to the substrate table. An electrostatic chuck comprises of one or more electrodes. On top of the electrodes a layer of dielectric material is placed. The top surface of this layer is usually patterned to reduce the contact area between substrate and chuck. A potential difference between the electrode and the substrate (mono-polar design) or between the two or more electrodes in the chuck itself (bi-polar and multi-polar design) is set to generate an electro-static force to clamp the substrate to the chuck.
Typically, the dielectric will have a polished smooth surface. However, in a practical situation the surface of the substrate and the dielectric will never be perfectly smooth and will contain small variations. Furthermore, although the resistivity of the dielectric is extremely high (greater than 108 Σm) a small leakage current will flow. This leads to the creation of Johnsen-Rahbek (JR) forces additional to the capacitive electrostatic force. These forces arise due to the flow of the leakage current between points where the substrate and dielectric are in contact. At points where the substrate and dielectric are not in contact, charge builds up and creates an additional JR force. When the potential difference is removed, the stored charge creating the JR force takes time to dissipate, and introduces a delay before the substrate can be released from the chuck. It has been proposed to apply a reverse voltage in the release stage in order to dissipate the charge more quickly. It has also been proposed to cycle the polarity of the applied voltage to minimize the charge build-up. However a relatively high cycle frequency is required and complicated schemes are required to maintain the clamping force at all times.
Accordingly, it would be advantageous to provide a chuck in which the Johnsen-Rahbek effect may be minimized, allowing greater throughput of substrates.